Photon counting detector based edge reference detector design and calibration method for small pixelated photon counting ct

ABSTRACT

An apparatus and a method for correcting for signal variations in pixels of a main photoelectric conversion element in a radiation detection apparatus due to focal spot position drifts. Edge reference detectors are positioned next to the main detector, in the fan beam coverage but outside the scan field of view. The signal variations of the edge reference detector pixels under the ant-scatter grid shadow are used to estimate the real-time focal spot movement, which is used to estimate the shadow/signal variation on the main detector pixels that are in the scan field of view.

TECHNICAL FIELD

The disclosure relates to a radiation detection apparatus used inmedical imaging.

BACKGROUND

For a typical scintillator detector-based conventional computedtomography (CT) system imaging, the X-ray tube emits certain amount ofphotons during an exposure to the scanning object, and a detector arrayon the other side of the scanning object measures the transmittedphotons, and then the measurement is normalized to an air scan at thesame scan setting without the scanning object to estimate theattenuation of the path length. Therefore, the air scan and the objectscan are taken place at a different time, so any variation in theincident X-ray beam in the time domain needs to be calibrated foraccurate measurement that leads to good image quality.

To achieve a good calibration of the X-ray tube flux variation overtime, typically a scintillator-based energy integrating detector (EID)is installed next to the beam exit to monitor the real time X-ray tubeflux change, and used as a normalization factor between scans. However,other than the X-ray tube flux change, the focal spot (FS) position alsodrifts more or less, depending on the tube type, over time due to theinternal electrical steering variation and anode thermal expansion, aswell as other design tolerances. Such positional variation usually wouldcause a random anti-scatter-grid (ASG) shadow profile change on theindividual detector pixels, and changing the measured intensity fromtime to time. Such FS positional variation combined with non-ideal ASGangular alignment can cause different intensity drifts across thedetector pixels, and result in ring artifacts in the reconstructedimage. On the other hand, the ASG may also experience certaindeformation due to high rotation speed, and cause positional androtational speed dependent intensity variation across the pixels.

There could be different ways to overcome these issues: 1) one way is toleave a certain inactive detector area in each pixel to allow such ASGshadow variation either due to the FS movement (right) or the ASG platedeflection (left) without affecting the intensity measurement (see FIG.1), 2) or improve the ASG alignment accuracy to allow a good cancelationfrom pixel to pixel (e.g. intensity increase or decrease by the sameamount) (see FIG. 2).

FIG. 1A and FIG. 1B show a type of detector design with inactive area ateach pixel edge to prevent intensity shift caused by ASG angulardeflection or FS movement, respectively. However, as a compromise, thisapproach also decreases the geometric detection efficiency.

FIG. 2 shows a detector pixel design without inactive areas. Itillustrates how the ASG shadow changes with FS movement, and theintensity variation across detector pixels with different ASG platetilting angles with respect to the nominal angles.

The dash lines indicate the nominal focusing angle for the individualASG plates. The solid lines indicate the projected shadow boundarieswith two different FS positions along the channel direction. Themeasured intensities of pixel 1 and 2 will decrease when FS moves fromposition 0 to position 1, but the intensity of pixel 4 will increase asthe shadow area changes in the opposite direction, and pixel 3 remainsthe same.

Without a proper correction, these intensity variations across pixelswould cause different normalization error for the individual pixel whenthe air scan and the object scan are taken with different FS locations,hence, generating ring artifact in the image.

For a semiconductor (CdTe/CZT)-based photon counting CT (PCCT), thetypical detector array design usually has a much smaller pixel sizecompared to the conventional CT detector, due to the trade-off betweencharging sharing effect and the pulse pile up effect to achieve the bestenergy resolving performance. Typically, the pixel pitch is chosenbetween 250 μm and 500 μm in one dimension, compared to ˜1 mm for theconventional pixel pitch. Thus, the conventional detector pixel area isusually equivalent to a N×N group of sub-pixels in PCCT, where N can bebetween 2 to 4. To maintain high dose efficiency, the ASG design usuallystill remains in the same pitch/spacing as the conventional system pixelpitch (see FIG. 3).

One important application for the PCCT is spectral imaging. To achievegood performance, accurate tube spectrum information is needed to solvethe material decomposition problem. Current EID reference detectors atthe tube side only monitor the total tube flux and may not be sensitiveto the spectrum change over time as the tube performance changes.Therefore, new reference detector design is also needed for the incidentspectrum monitoring/calibration purpose.

For a small pixelated photon counting detector (PCD) design, the ASGplates usually keep the same spacing as the conventional CT design asillustrated in FIG. 3. A 3×3 sub-pixel scheme is used as an example, forwhich each sub-pixel is ˜⅓ of the conventional detector pixel size.

In such a design, the ASG shadow now only affects sub-pixel 1 and 3 ineach group, and the middle sub-pixel 2 is not affected by those effectsas previously described. Therefore, even with a perfect ASG platealignment, the sub-pixel readout would always have normalization erroracross the detector along the FS movement direction, and this is arandom correction factor that no existing apparatus can resolve. Thiswould generate ring artifact in the high resolution images which use thesub-pixel level readout for image reconstruction.

In reality, the ASG plates always have certain mechanical tolerances forboth positional accuracy and angular accuracy. Therefore, the combinedreadout (e.g., 3×3 summing mode) would also encounter the same problemas described in FIG. 2, and generate ring artifact in the standardresolution images which use the combined pixel readout forreconstruction when this effect is significant enough.

In a PCCT project measurement, one typically measures 2-6 energy bincounts. As an example, for detector pixel i, the measured 5 bin countscan be modeled as the following:

${N_{bi}\left( {{b = 1},\ldots\mspace{14mu},5} \right)} = {\int_{T_{b}}^{T_{b + 1}}{{\Phi_{b}\left( E^{\prime} \right)}{\int_{E\min}^{E\max}{N_{0i}{S_{0i}(E)}{D\left( {E,E^{\prime}} \right)}e^{\sum_{j = 1}^{J}{{\mu_{j}{(E)}}l_{j}}}d{EdE}^{\prime}}}}}$$\mspace{20mu}{{\Phi_{b}(E)} = \left\{ \begin{matrix}{1,} & {T_{b} \leq E \leq T_{b + 1}} \\{0,} & {others}\end{matrix} \right.}$

N_(0i) is the incident flux determined by using the air scan without thescanning object. Any tube flux variation can be captured and correctedby the tube side reference detector. But the air scan flux variation canbe also due to the focal spot related movement as previously explained,and this cannot be captured by the reference detector readout (Ref) atthe tube side, therefore introduces error in using this forward model toestimate the material path lengths:

$\frac{N_{bi}{\_ ref}}{N_{0i}{\_ ref}} = {\int_{T_{b}}^{T_{b + 1}}{{\Phi_{b}\left( E^{\prime} \right)}{\int_{E\min}^{E\max}{{S_{0i}(E)}{D\left( {E,E^{\prime}} \right)}e^{\sum_{j = 1}^{J}{{\mu_{j}{(E)}}l_{j}}}{dEdE}^{\prime}}}}}$${N_{bi}{\_ ref}} = \frac{N_{bi}}{{Ref}_{obj}}$${N_{0i}{\_ ref}} = \frac{N_{0i}}{{Ref}_{air}}$

N_(bi)_ref is the reference reading corrected bin count measurement withscanning object, and N_(0i)_ref is the reference reading corrected airscan. Ref_(obj/air) is the reference detector reading for the object/airscan. The reference detector reading at the tube side is not sensitiveto the focal spot movement related flux change on the main detector.

In addition, the above forward model requires accurate incident spectrumS_(0i) (E) as a known input, and any drift of this spectrum over timewithout knowing also introduces error in the estimated path lengths andgenerates bias in the reconstructed image.

SUMMARY

In order to monitor the FS movement-induced ASG shadow shift, andcorrect the associated detector pixel intensity variation, a newPCD-based edge reference detector design with an extended ASG covered ontop of the edge detector pixels is presented herein (FIG. 4).

The edge reference detectors need to be in the fan beam coverage butoutside the scan field of view (FOV) so that the measurement is notaffected by the change of scanning path length. The edge referencedetector comprises at least one group of sub-pixels with a N×N pattern,where N>=3. Therefore, the middle pixel(s) are not affected by the ASGshadow effect.

The signal variations of the edge reference detector pixels under theASG shadow are used to estimate the real-time FS movement, which is usedto estimate the shadow/signal variation on the main detector pixels thatare in the scan FOV.

Depending on the FS movement speed, the estimated variation is correctedon each view, or on a group of views. The correction is applied on boththe sub-pixel level readout and the combined-pixel mode readout.

With a two dimensional (2D) ASG design on the main detector, one canapply this correction on both the channel direction and the rowdirection FS movement.

Using the PCD, the edge reference detector described herein also hasmultiple energy bin measurements to monitor the tube spectrum variation.

BRIEF DESCRIPTION OF THE DRAWINGS

The application will be better understood in light of the descriptionwhich is given in a non-limiting manner, accompanied by the attacheddrawings in which:

FIG. 1A shows a schematic of a detector design with an inactive area ateach pixel to prevent intensity shift caused by ASG angular deflection.

FIG. 1B shows a schematic of a detector design with an inactive area ateach pixel to prevent intensity shift caused by FS movement.

FIG. 2 shows a schematic of a detector pixel design without an inactivearea.

FIG. 3 shows an example of ASG design with a small pixelated PCD.

FIG. 4 shows a schematic of an edge reference detector design.

FIG. 5 shows a cross section of scintillator-based EID.

FIG. 6 shows a schematic of edge reference detector correction workflow.

FIG. 7A shows a schematic of shadow caused by FS movement in channeldirection with a 1D ASG.

FIG. 7B shows a schematic of shadow caused by FS movement in channeldirection with a 1D ASG with higher plates. The shadow is biggercompared to the design in FIG. 7A with the same FS movement.

FIG. 8 shows a schematic of an edge reference detector covered by 1DASGs.

FIG. 9 shows a schematic showing the ASG shadow influence on twoneighboring pixels with non-ideal ASG-FS alignment.

FIG. 10A shows a schematic of a main detector with a 2D ASG.

FIG. 10B shows a schematic of an edge reference detector with two 1DASGs.

FIG. 11 shows a schematic of an edge reference detector with a 2D ASG.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, material, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the application, but do not denote thatthey are present in every embodiment.

Thus, the appearances of the phrases “in one embodiment” or “in anembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the application.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

A scintillator-based EID is shown in FIG. 5. It comprises a grid 1 thatincludes radiation-absorptive (e.g., Pb) members 11 andradiation-transmissive (e.g., Al) members 12 alternatively arranged inthe form of slits or a matrix. The members can be one-dimensional (1D)or 2D. The grid is positioned on substrate 2 and photoelectricconversion unit 3 having pixels 101, which is arranged on scintillator4.

A PCD-based edge reference detector design with an edge ASG covering thetop of the edge detector pixels is shown in FIG. 4.

In one embodiment, as shown in FIG. 4, a small section of the PCD pixelsare located at the edge of the main PCD array. A small portion of theASG, same as, or different from the ASG on the main detector are mountedon those edge PCD pixels using a N×N (N_(>)=3) pixel group pitch,focusing to the FS. To avoid pileup effect which will skew the fluxmeasurement, a piece of beam attenuator with appropriate attenuationlength is added at the beam exit to make sure the measurement of thereference detector is at low flux condition. The beam attenuator may bemade of common attenuation materials like Al, Cu, Ti, etc. This can be apart of or an extension of the bowtie filter that shapes the beamprofile on the main detector.

As shown in FIG. 4, the PCD based edge reference detector is locatedoutside the scan FOV to provide real-time monitoring of the FS movementas well as the tube spectrum variation. An extension of the maindetector ASG or a different ASG is needed to cover the edge referencedetector. As an alternative, different ASG patterns can be used ondifferent sections of the detector to monitor the FS movement in boththe channel and row direction if a 2D ASG is used for the main detector.

FIG. 6 shows the main workflow of the edge reference detectorcorrection.

During scans, the edge reference detector will always readoutsimultaneously with the main detector, and used for data processing. NoASG scans would be needed to measure the pixel uniformity map for boththe main detector and the edge reference detector, and used asnormalization factors for the individual pixels to estimate the ASGalignment. Then, the intensity (e.g. total counts) variation over thescan can be used to estimate the ASG shadow change, then using thegeometric information to estimate the FS movement along the orthogonaldirection of the ASG plate orientation.

For the pixels (central ones) that are not under the ASG shadowinfluence, the signal change can be used to monitor the tube fluxvariation over time, similar to the conventional EID reference detectorat the tube side. They can also monitor the tube spectrum variation withmultiple energy bin measurements. With the estimated FS movement, thecorresponding intensity drifts between the air scan and the object scanon the main detector pixels can be estimated and corrected.

In order to estimate the FS movements from the edge reference PCD,various designs may be selected.

In one embodiment, the same ASG design (height, thickness and spacing)as the one on the main detector is used (FIG. 7A).

In another embodiment, a higher ASG plate with the same thickness andspacing as the main ASG is used to enhance the measurement sensitivityto the FS movement (FIG. 7B). As the estimated FS drift in the channeldirection is approximated as:

$\begin{matrix}{{D_{fs} = {L_{shadow}*\frac{L_{SAD}}{L_{ASG}}}},{{{{with}\mspace{14mu} L_{SAD}} ⪢ L_{ASG} ⪢ t};}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

Therefore, with larger L_(ASG), the shadow caused pixel intensity changeis more significant and gives more accurate FS movement estimate withthe same measurement statistics.

The same concept also applies for a 2D ASG design, and the FS movementin the row direction can also be estimated using the same formula when a1D ASG along the other direction is used, see Eq. 1.

The measured intensity, in this case, the total counts of the edgereference detector pixels are used to estimate L_(shadow). Using a 1DASG design as an example, one method is based on the linearapproximation ignoring the charge sharing/cross talk effect betweenneighbouring pixels:N≈N₀(L_(pixel)−x_(asg)−L_(shadow))/(L_(pixel)−x_(asg)), where L_(pixel)is the pixel size, x_(asg) is the initial ASG shadow which is

$\frac{t}{2}$

with ideal ASG-pixel alignment, and L_(shadow) is the additional shadowcaused by non-ideal FS-ASG alignment, and in this case, by FS movement.

In reality, x_(asg) can deviate from

$\frac{t}{2}$

due to ASG alignment tolerance (FIG. 2), as well the deflection undergantry rotation, and one can estimate the initial shadow x_(asg) basedon the pixel intensity difference between neighboring pixels afternormalizing with the no ASG measurements (detector uniformity map). Onemethod is to compare the normalized intensity of the ASG covered pixelswith the uncovered ones to estimate x_(asg)

$\begin{matrix}{{\frac{N_{ASG}}{N_{0}} = {1 - {x_{asg}/L_{pixel}}}},} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where N_(ASG) is the normalized ASG covered pixel intensity, and N₀ isthe normalized uncovered pixel intensity. x_(asg) is rotation speeddependent, and this measurement needs to be taken for every availablerotation speed for correction, see FIG. 8 for a demonstration for the 1DASG populated at channel direction. The ASG covered pixels are marked asA, and the uncovered ones are marked as B. The dashed boxes indicate themisalignment of ASG from its ideal location (solid boxes).

A variation of the method to minimize the effect from ASG alignmenttolerance and estimate the shadow is to use the sum of two neighboringedge pixels that are under the ASG septa, assuming the chargesharing/cross talk effect is 0 between these two pixels:

(N _(R) ^(i) +N _(L) ^(i+1))=2N ₀(2L _(pixel) −t−L _(shadow))/(2L_(pixel))  (Eq. 3)

A variation of the method can further include the charge sharing/crosstalk effect between the neighboring pixels assuming the chargesharing/cross talk effect is proportional to the boundary length betweenthe pixels to further improve the estimation accuracy.

The new reference normalized air scan and object scans are given by:

$\begin{matrix}{{{N_{0i}{\_ ref}} = {\frac{N_{0i}}{{Ref}_{air}}*f_{0i}{\_ shadow}}},{{N_{bi}{\_ ref}} = {\frac{N_{bi}}{{Ref}_{obj}}*f_{i}{\_ shadow}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

Where, f_(i)_shadow is the additional shadow correction factor for themain detector pixel i based on the estimated FS drift D_(fs) from theedge reference detector measurement. An alignment factor m_(i) is addedto account for the initial orientation of the ASG plates with respect tothe FS position, and is either 0 or 1 (see FIG. 9). m_(i) can bedetermined by comparing the detector intensity variations between aseries of air scans that cover the full FS position range.

$\begin{matrix}{f_{i_{shadow}} = \frac{L_{main} - {m_{i}*D_{fs}*\frac{L_{{ASG}{({main})}}}{L_{{SAD}{({main})}}}}}{L_{main}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

With reference to FIG. 9, the ASG shadow influences on two neighboringpixels with non-ideal ASG-FS alignment. m₁=0, m₂=1 when FS position isbetween 0 and 1; m₁=1, m₂=0 when FS position is between 0 and 2. Due tothe ASG plates alignment variations, the 0 position could be differentfor the pixels under ASG, and can be estimated through a few air scansthat cover the full FS movement range.

This correction can be applied on different rotation speed, as anadditional correction to the air normalization which includes the ASGdeflection variation on the main detector at different rotation speed.

In order to make sure that the edge reference PCD is at low flux regionto avoid complications due to pulse pileup, one can employ multiple beamattenuator designs with different attenuation lengths to cover the fulloperational flux range.

When a mA/kV setting is selected for the scan, the optimal beamattenuator is pre-selected and put in position for those edge referencePCDs to make sure the measurement satisfies the low flux condition withsufficient statistics for an accurate estimation.

The low flux condition can be defined as nτ<0.05, where n is the pixelcount rate, and τ is the ASIC dead time for processing one signal pulse.The appropriate length of the attenuator can be theoretically calculatedand designed based on the material's attenuation coefficient and thesimulated tube spectrum.

If a 2D ASG design is used on the main detector (FIG. 10A), the edgereference detector can use two 1D ASGs, one on the channel direction andthe other on the row direction at different locations, to estimate theFS movement on each direction separately (FIG. 10B).

A variation of the design for 2D ASG on the main detector is to use thesame or a different 2D ASG design on the edge reference detector aswell.

In an example of a 3×3 sub-pixel group for the edge detector (FIG. 11),pixels 2 and 8 are only under the row direction ASG shadow while pixel 4and 6 are only under the channel direction ASG, therefore, they can beused respectively to estimate the FS movement on both directions.

In conventional CT systems, the reference detectors are typicallyscintillator-based energy integrating detectors, and located at the tubeside. The PCD based edge reference detector design described herein canprovide the FS position information as well as the tube spectruminformation, which are crucial for a small pixelated PCCT measurementand the resulted image.

In the design in FIG. 1, as the inactive area is introduced for everypixel, the dose efficiency is compromised, compared to the designdescribed herein.

Obviously, numerous modifications and variations of the embodimentspresented herein are possible in light of the above teachings. It istherefore to be understood that within the scope of the claims, thedisclosure may be practiced otherwise than as specifically describedherein.

1-21. (canceled)
 22. A photon counting CT apparatus comprising: a photoncounting detector (PCD) arranged in a two-dimensional (2D) array ofpixels; and an anti-scatter-grid (ASG) arranged associated with the PCDand configured to remove scattered radiation, wherein the PCD comprisesa main detector positioned within a field of view (FOV) and a referencedetector positioned outside the FOV.
 23. The photon counting CTapparatus of claim 22, wherein the photon counting CT apparatus furtherdetermines focal spot movement of an X-ray tube based on a countdetected by the reference detector.
 24. The photon counting CT apparatusof claim 23, wherein the photon counting CT apparatus determines thefocal spot movement based on, among counts detected at a plurality ofpixels included in the reference detector, a first count detected at afirst pixel which is influenced by a shadow of the ASG and a secondcount detected at a second pixel which is not influenced by the shadowof the ASG.
 25. The photon counting CT apparatus of claim 24, whereinthe photon counting CT apparatus determines a length of the shadow ofthe ASG based on the first count and the second count, and determinesthe focal spot movement based on the length of the shadow.
 26. Thephoton counting CT apparatus of claim 22, wherein the reference detectorcomprises at least one group of pixels with an N×N pattern, where N≥3.27. The photon counting CT apparatus of claim 22, wherein the ASGcomprises a first ASG covering the main detector and a second ASGcovering the reference detector, and the second ASG has a height greaterthan a height of the first ASG.
 28. The photon counting CT apparatus ofclaim 27, wherein the first ASG is a two-dimensional ASG, and the secondASG comprises two one-dimensional ASGs, one of the two one-dimensionalASGs on a channel direction and the other on a row direction.
 29. Thephoton counting CT apparatus of claim 22, further comprising multiplebeam attenuators with different attenuation lengths provided between anX-ray tube and the reference detector.
 30. The photon counting CTapparatus of claim 23, the photon counting CT apparatus furthercorrects, based on the focal spot movement of the X-ray tube, a countdetected by the main detector.
 31. The photon counting CT apparatus ofclaim 30, wherein the photon counting CT apparatus corrects a countdetected based on the focal spot movement in a channel direction and acount detected based on the focal spot movement in a row direction, eachdetected by the main detector.
 32. The photon counting CT apparatus ofclaim 25, wherein the length of the shadow is a first length indicatingvariation in the length of the shadow created by the ASG upon movementof the focal spot of the X-ray tube from a first position to a secondposition, and the photon counting CT apparatus determines the firstlength based on the first count, the second count, and a second lengthwhich is a length of a shadow created by the ASG when the focal spot ispositioned at the first position.
 33. The photon counting CT apparatusof claim 25, wherein the photon counting CT apparatus determines thefocal spot movement based on the length of the shadow and a height ofthe ASG covering the reference detector.
 34. The photon counting CTapparatus of claim 22, wherein photon counting CT apparatus furtherestimates variation in an amount of incident on the PCD based on a countdetected by the reference detector.
 35. The photon counting CT apparatusof claim 34, wherein the photon counting CT apparatus further correctsthe amount of incident on the PCD based on the estimated variation inthe amount of incident.
 36. A method performed by a photo counting CT,the photo counting CT comprising: a photo counting detector (PCD)arranged in a two-dimensional (2D) array of pixels, and configured tocomprise a main detector positioned within a field of view (FOV) and areference detector positioned outside the FOV; and an anti-scatter-grid(ASG) arranged associated with the photo counting detector andconfigured to remove scattered radiation, the method comprising:determining a length of a shadow created by the ASG based on, amongcounts detected at a plurality of pixels included in the reference PCD,a first count detected at a first pixel which is influenced by a shadowof the ASG and a second count detected at a second pixel which is notinfluenced by the shadow of the ASG; and determining focal spot movementof an X-ray tube based on the length of the shadow.